Existing 2G and 3G cellular systems such as Global System for Mobile Communications (GSM) and Universal Mobile Telephone System (UMTS) operate over a frequency band which is relatively narrow compared to the frequency of operation—for example, the UMTS system has an operating band extending from 1920 to 2170 MHz. The design of antennas offering good performance with bandwidths for one or more 2G or 3G systems is relatively well established.
Future wireless networks will be required to provide much higher data transfer rates than existing systems, and as a result the required operating bands will generally become wider. The UWB systems defined by the WiMedia Alliance and the IEEE 802.15.3 standards describe systems with operating bands ranging from 3.2 to 10.6 GHz. At the same time, the future evolution of wireless handsets and terminals will see an increased functionality and the capability to operate on multiple systems, so that the physical dimensions of the constituent parts of each system will become necessarily smaller. For such future systems, a new type of antenna design becomes an imperative: an antenna which retains the small physical dimensions of antennas for 2G and 3G systems while offering good performance over a bandwidth extending over several GHz.
Wideband planar antennas are well known; for example, U.S. Pat. No. 5,828,340, Johnson, describes a planar antenna having a 40% operational bandwidth, where the extended bandwidth is achieved by forming a tab antenna on a substrate where the tab antenna has a trapezoidal shape. Furthermore, it is known that the physical dimensions of an antenna can be reduced by fabricating the antenna on a substrate with a high dielectric constant, such as Alumina. U.S. Pat. No. 7,019,698, Miyoshi, describes a gap-fed chip antenna comprising a radiating portion formed by the union of a reversed triangular portion and a semicircular portion sandwiched between two dielectric layers and comprising a feeding portion which couples to the radiating portion. The antenna taught by Miyoshi is suitable for use as an antenna device operating according to the UWB system and has dimensions in the order of one quarter of one wavelength at an operating frequency of 6 GHz. A similar antenna is described in U.S. Pat. No. 7,081,859, Miyoshi et al.
FIG. 1 shows a prior art monopole chip antenna comprising a dielectric chip 10, arranged on an insulating carrier substrate 15. The antenna includes a radiating structure 11 fabricated on an obverse face of dielectric chip 10, a feed point, realized by a metal input/output (I/O) pad 12 fabricated on carrier substrate 15, and a corresponding device terminal fabricated on a reverse face of dielectric chip 10. A metal connecting trace 16A connects I/O pad 12 to radiating element 11. Carrier substrate 15 includes a feed line 17 which connects a transceiver device (not shown) to metal I/O pad 12.
Despite the advances taught in Johnson and Miyoshi, for integration in mobile wireless handsets and terminals, antennas with further reduced physical dimensions are highly desirable. Moreover a solution to the problem of producing a highly miniaturized ultra wideband antenna with excellent performance characteristics (e.g. a return loss of less than −6 dB and a high radiation efficiency over a frequency range from 3.2 to 10.6 GHz) has, so far, yet to be found.
Accordingly, it would be desirable to provide a wideband chip antenna fabricated on a dielectric substrate, which is suitable for integration in a portable wireless handset or terminal, where the bandwidth of the antenna extends over an ultra wide band frequency range, e.g. from 3.2-10.6 GHz, and where the antenna has dimensions which are small compared with the wavelength of the lower edge of the operating frequency band of the antenna.
FIG. 11 shows the band groups of the UWB system as defined by the WiMedia Alliance. It can be seen that frequency range extends from 3.2 GHz to 10.6 GHz.
It is widely accepted in industry that any service offering data transfer using by the UWB system will not use UWB band group 2, since sections of UWB band group 2 have already been allocated to the 802.11a system. It is acceptable therefore for the antenna to exhibit a poor response over the frequency range of the 802.11a because this eases the specifications for RF filters required to block 802.11a signals from the UWB front-end. Accordingly, it would be desirable to provide an antenna wherein the frequency response can be tuned to take advantage of system characteristics such as that described above.